Electrically conductive compositions and coatings are well known in the art. Processes for forming such compositions, and details of general applications of same are described, for example, in U.S. Pat. No. 3,099,587, wherein electrically conductive coatings comprising a mixture of C, Ag, and Fe in a thermosetting resin are applied to non-conductive surfaces. Due to the expense of precious metals, large scale commercial use of these coatings is unattractive.
In another example, electrothermic coatings are described in British Patent No. 1,286,855 and comprise aqueous mixtures of graphite and conductive carbon black in potassium silicate and lithium silicate-sodium solutions along with appropriate wetting agents. However, the coatings require use of large expensive transformers for voltage control when the coatings are used as paint for radiant heating in homes. Thus installation costs for such applications are prohibitively high.
U.S. Pat. No. 5,545,679 describes positive temperature coefficient conductive polymers made form thermosetting polyester resins and conductive fillers. The conductive fillers are chosen from carbon and/or graphite. These compositions are useful in current limiting devices. For example, when the current increases due to short circuit or overload, the composition temperature increases and changes to a high resistance state which limits the current to a safe value until the power is removed. When the power is removed, the material returns to its original low resistance state, and is therefore a resettable fuse or current limiter device.
U.S. Pat. No. 6,086,791 describes electrically conductive electrothermic paints that resist breakdown for several months, while outputting relatively constant heat when alternating current (AC) is applied. The coating compositions comprise a heat resistant binder, conductive carbon, non-conductive flake-like carbon black pigment particles and conductive flake-like graphite pigment particles of particle sizes of from about 5 to 500 microns.
U.S. Pat. No. 6,818,156 describes the preparation of heat emitting coating compositions. These coating compositions comprise curable binders, and electrically conductive carbon black particles generated by high temperature pyrolysis of acetylene at around 8000° F. (Ketjenblacks) and which have particle sizes between about 0.001 and 500 microns. The document teaches at column. 2, that use of the acetylene-based carbon black particles alone will not result in a stable coating system. That is, the coating will continue to heat until it burns out (i.e., breaks down). An electrically conductive graphite particle having similar particle size to the carbon black is also used. The document also teaches that use of graphite with a lesser electrical conductivity is preferred, and that crystallinity of greater than about 67% is also preferred.
In addition to the above-mentioned general problems associated with prior art coatings, performance issues make large scale commercial use not practical. For example, heat output can be difficult to control, and prior art electrothermic coatings or paints can show great variation in their day-to-day electrical conductivity and heat generation.
It would be desirable to provide alternative electrothermic compositions that are highly conductive, self regulating, do not “run away”, do not deteriorate over long periods of time, and can be formed into useful products, such as surface coatings or even 3D shapes.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.